Apparatus for measuring harmonic distortion in a conductor, and methods of constructing and utilizing same

ABSTRACT

A portable instrument for locating and measuring harmonic distortion in a conductor. The instrument permits harmonics to be easily read while the load is changing. Thus, the instrument may provide a great deal of insight as to the source of problem causing harmonics.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention pertains to apparatus for measuring and indicatingharmonic distortions in conductors, such as power lines. Moreparticularly, the present invention pertains to a portable,simple-to-use, and highly versatile instrument for quickly measuring andindicating a range of individual harmonic distortions in conductors.

2. Description of the Relevant Art

There are known devices for detecting and/or displaying harmonicdistortion in conductors. Such known devices are for example, disclosedin U.S. Pat. No. 2,782,366, 3,916,296, and 3,927,281.

U.S. Pat. No. 2,782,366 discloses "oscillographic apparatus foranalyzing the performance of amplifiers, modulators, detectors and thelike." The disclosed apparatus comprises a circuit including a testoscillator, a group of potentiometers, amplifiers tuned to first, secondand any other desired harmonic, a battery, and a group of oscilloscopesfor providing output displays. This known apparatus has manydisadvantages. For example, it is very bulky and difficult to use,especially in remote locations. Also, this apparatus can only be used intesting individual parts which have been disconnected/isolated fromtheir normal use, and harmonic distortion in voltage signals.

U.S. Pat. No. 3,916,296 discloses a high sensitivity/high resolutiondistortion analyzer including a parallel-T circuit bridge that is tunedby adjusting two variable elements, typically a resistor in one branchof the circuit and a capacitor in a second branch of the circuit. Theturning is achieved by separate feed-back circuits that utilize a +45°or a -45° phase-shifted signal derived from an input voltage signal.This apparatus is substantially easier to use than that disclosed inU.S. Pat. No. 2,782,366, but still has several problems anddisadvantages associated therewith. For example, the apparatus is stillquite inconvenient to use (especially in remote locations) because theoperator is required to turn off the power to an electrical componentbefore connecting the analyzer thereto (resulting in expensive downtime), and because the operator is required to adjust the variableelements. Also, the disclosed analyzer, like that disclosed in patentU.S. Pat. No. 2,782,366, is limited to analyzing the harmonicdistortions of a voltage signal, and does not include any means forselectively choosing various single harmonics to be measured.

U.S. Pat. No. 3,927,281 discloses an instrument for measuring harmonicdistortion in telephone transmission lines. The instrument measures theharmonic distortion introduced during telephone line transmission, andcan distinguish between the harmonic distortion introduced at the sendterminal and that introduced in the receiver terminal The disclosedinstrument comprises a frequency phase lock circuit which generates"disturbance-free", in-phase and quadrature voltage signals of areceived test tone, the disturbance-free generated tones being differentfrom the transmitted test tone as a result of possible frequency shiftalong the transmission channel and phase shifts in the terminalequipment at both ends. As disclosed the instrument is connected to anoscilloscope 50 for providing an output display. This instrument hasassociated therewith substantially the same problems and disadvantagesas discussed above with regard to the analyzer disclosed in U.S. Pat.No. 3,916,296.

Conventional distortion analyzers, including those discussed above, havemany problems and disadvantages associated therewith, and have as awhole failed to fulfill a need in the art for a portable, simple-to-useand highly versatile instrument which can be used for quickly and easilylocating and measuring problem-causing harmonic distortion insubstantially any conductor while the conductor is in normal operation.

SUMMARY OF THE INVENTION

The present invention has been designed to overcome the many problemsand disadvantages of known harmonic distortion analyzers, and to satisfya great need in the art, as discussed above.

According to the present invention, there is provided an apparatus forlocating and measuring harmonic distortion in a conductor, the apparatuscomprising a first means detachably connectable to a conductor forproviding an input signal indicative of electrical energy in theconductor, a second means connected to the first means and receiving theinput signal therefrom, the second means providing a modified signalindicative of the strength of a single, predetermined harmonic of theinput signal, and third means connected to the second means forselectively varying the single, predetermined harmonic which isindicated by the modified signal.

Additionally, the apparatus according to the present invention mayfurther comprise a fourth means connected to the third means andreceiving the modified signal therefrom, the fourth means providing adisplay of the modified signal, and a fifth means interconnected betweenthe first and second means for automatically controlling a gain of theinput signal so that the input signal remains essentially constantdespite variations of the electrical energy in the conductor.

It is an object of the present invention to provide a portable,simple-to-use and highly versatile instrument for measuring a range ofindividual harmonic distortions in electrical conductors, and forreadily locating sources of such harmonic distortions.

Another object of the present invention is the provision of such aninstrument which can be readily attached to conductors, such as powerlines, while electrical energy is continuously present therein, or inother words, without circuit interruptions.

A further object of the present invention is the provision of such aninstrument capable of producing accurate harmonic measurements andindications over very large voltage and current ranges.

Still another object of the present invention is the provision of a fastacting auto-gain circuit acting on an input signal from the first meansto thereby eliminate the need for manually zeroing the instrument ascircuit loads vary, and so that harmonics can be easily and accuratelyread while the amount of electrical energy in the conductor is changing.

Yet another object of the present invention is the provision of such aninstrument which is very compactly and efficiently arranged in animpact-resistant, water-resistant case such that an operator can easilycarry and manipulate the instrument with a single hand.

Other objects, advantages and salient features of the present inventionwill become apparent from the following detailed description, which,when taken into conjunction with the annexed drawings, discloses apreferred embodiment of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a portable instrument according to thepresent invention.

FIG. 2 is a block diagram of a main circuit of the instrument shown inFIG. 1.

FIG. 3 is a block diagram of an internal, rechargeable power supply ofthe instrument shown in FIG. 1.

FIG. 4 is a schematic diagram of the main circuit for the instrument.

FIG. 5a is a schematic diagram of the power supply circuit, and FIG. 5bis a schematic diagram of a low battery indicator circuit for the powersupply.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, there is shown a portable instrument according tothe preferred embodiment of the invention, which is generally indicatedby reference numeral 1. The instrument includes a carrying case 2 whichinternally houses the main circuit 4 shown in FIG. 4, the power supplycircuit 6 shown in FIG. 5a and the low battery indicator circuit 9 shownin FIG. 5b; a clamp on current transformer 8 which is selectivelyconnectable to the main circuit 4 through a signal input jack 32; apower on/off switch 10 and a corresponding power on indicator light 12;a display scale 14 for indicating the level of harmonic distortion; aselector switch 16 which permits an operator to select a particularharmonic which is to be shown on the display scale 14; a low batteryindicator light 18; a battery charging cord 20 and a corresponding powerinput jack 22.

The case 2 is preferably constructed from a moldable, impact-resistant,water-resistant plastic, and has the circuits 4, 6, 9 permanentlymounted in a base portion 24 thereof below a face plate 26. The powersupply circuit 6 preferably includes the pair of rechargeable batterypacks 28, 29 (See FIG. 5a), which battery packs would be recharged whennecessary using the removable charging cord 20. However, circuit 6 couldhave any other appropriate/desired construction. For example, thecircuit 6 could include a single battery pack and a voltage inverter.

The current transformer 8 is selectively connectable to the main circuit4 through a signal cord 30 which plugs into the signal input jack 32 onthe face plate 26. The depicted, preferred current transformer 8 is aconventional, readily available component constructed as two halves 34,36, each including a portion of the transformer core 35 at one endthereof. The halves 34, 36 are pivotally connected together such thatthe portions of the transformer core 35 can be opened by manuallysqueezing together a pair of handles 38, 40 at opposite end of thehalves 34, 36 thereof. The current transformer 8 also includes a biasingmeans (not shown) for normally holding the core end of the transformerin a closed position. Then, the current transformer 8 can be easilyattached to/around substantially any conductor by simply squeezing thehandles 38, 40 together, positioning the opened end of the currenttransformer next to the conductor, and releasing the handles so that theopened end of the transformer clamps around the conductor. Thus, thecurrent transformer can be clamped around the conductor withoutinterrupting the circuit, thereby enabling measurements during plantoperation.

The current transformer 8 preferably has a frequency response to atleast 5 kilohertz so that higher harmonics are not attenuated and sothat the transformer provides an accurate input signal over a largemeasurement range. Also, the transformer is preferably insulated topermit measurements in bus bar systems energized up to very highvoltages, such as up to 600 volts AC.

The harmonic selector switch 16 permits an operator to choose theparticular harmonic which is to be measured by the instrument. Theselector switch is preferably constructed as a rotatable dial whichlocks into a plurality of positions corresponding to the variousharmonics. For example, is an preferred construction of the invention,the selector switch 16 could be rotated to eleven different positionscorresponding to current or voltage amplitude for the fundamental, 2ndharmonic, and odd harmonics 3rd through 19th. These harmonics are themost likely to cause problems in electrical power systems.

The display scale 14 provides output readings for the instrument, whichreadings show the current amplitude of the selected harmonic as apercentage of the fundamental current amplitude. As shown, the displayscale 14 is preferably constructed as a multi-step light bar and acorresponding printed scale reading 0% to 100%.

Optionally, the instrument 1 may also include a recording device (notshown), such as a paper chart and a marking tool, which could record theoutput readings over a period of time.

When the instrument 1 is not being used, or is being transported, thesignal cord 30 and the charging cord 20 are disconnected from theirassociated jacks 22, 32 and a lid 25 of the case 2 is secured over thebase 24 by latches 27. Also, a padded carrying case (not shown) may beused for conveniently carrying the case 2, the current transformer 8,the signal cord 30 and the charging cord 20 during transportation orperiods of non-use.

Referring to FIGS. 2 and 4 there is respectively shown a block diagramand a schematic diagram of the main circuit 4 of the instrument 1. Themain circuit 4 comprises an input signal conditioner 50 which receivesan input signal from the current transformer 8, auto-gain circuit 52which receives a conditioned input signal from the conditioner 50, abuffer 54, a filter 56 which filters the input signal and provides amodified signal indicative of the strength of a single predeterminedharmonic of the input signal (corresponding to the harmonic chosen usingthe selector switch 16) relative to the strength of a fundamentalharmonic of the input signal, a programmable clock or frequencysynthesizer 58 which includes the harmonic selector switch 16 and isconnected to the filter 56 for permitting the operator to selectivelyvary the single, predetermined harmonic which is indicated by themodified signal, an amplifier 60, and a display circuit 62.

According to the preferred embodiment of the present invention theinstrument 1 is used to measure a current input signal from the currenttransformer 8. However, it is within the scope of the present inventionthat the instrument 1 could alternately be used to measure the harmonicdistortions of a voltage input signal by connecting the main circuit 4between the conductor and ground. For such use, it would normally benecessary to provide a stepdown transformer between the conductor andthe main circuit 4 to assure a sufficiently low voltage input signal.Also, because voltage harmonics normally constitute a significantlysmaller percentage of the fundamental than corresponding currentharmonics, it would be desirable to modify the display circuit 62 anddisplay scale 14 so as to provide output readings over a smallerpercentage range of the fundamental, such as 0% through 10%. The inputsignal conditioner 50 shown in FIG. 2 is indicated as having both acurrent to voltage portion and a voltage to voltage portioncorresponding to a current input signal and a voltage input signal,respectively.

The auto-gain circuit 52 is constructed of fast acting, state of the artelectronic components which are initially factory calibrated, therebyeliminating any need for manually zeroing the meter before each use, oras the amount of electrical energy in the conductor varies. This featureof the present invention desirably permits harmonics to be easily readwhile the load is changing. Thus, data can be taken to show thedependence of a single harmonic or several harmonics upon changing loadconditions, and such data can provide a great deal of insight as to thesource of problem-causing harmonics.

The preferred embodiment of the auto-gain circuit 52 comprises a voltageto current converter or (rectifier) 64, a buffer 66, a current control,variable gain amplifier (or cell) 68 and an operational amplifier (opamp) 70. Note, there are available compandor chips, such as NE 571 whichcombines elements 64-70 and NE 572 which combines elements 64-68, thatcould be used in the present invention. The automatic gain circuit 52receives an input signal from the signal conditioner 50 and controls thegain of the input signal to provide a substantially constant outputsignal to the buffer 54. Rectifier 64 and op amp 70 receive power inputfrom the battery 28 at terminals 72, 74, respectively, while op amp 70is also connected to battery 29 at terminal 76. Buffer 54 is connectedto battery 29 at terminals 78, 80.

Filter 56 preferably, comprises a multiple-order programmable, switchedcapacitor filter 82, such as LTC 1061 CN, and an amplifier 84. Thefilter 82 would preferably be programmed as a band pass filter usingknown software, and would be connected to battery 29 at terminals 86,88. However, the filter 56 could comprise any other desired orappropriate means. For example, the filter 56 could be constructed as anactive filter using op amps.

The frequency synthesizer or programmable clock 58 could, for example,comprise a source clock 90, a phase lock loop 92, a pair of divide by nCMOS counters 94, 96, a programmable logic array (PAL) 98, the harmonicselector switch 16, and a pull-up resistor network 100 associated withthe switch 16. The source clock 90 is preferably a one kilohertz sourceclock, such as a PXO-1000 programmed as a one kilohertz source clock,while a 4046 chip could be used as the phase lock loop 92, 4522 chipscould be used as the counter dividers 94, 96, and an EPROM chip, such asTMS 2516, could be used as the PAL 98. Of course, any other desiredsource clock 90 could be used, in which case a different, appropriatePAL 98 might also have to be substituted for the TMS 2516. Components90, 92, 94, 96, 98 and 100 are each connected to the battery 29 atterminals 102, 104, 106, 108, 110, and 112, respectively. The filter 56is connected to the frequency synthesizer 58 through the output of thephase lock loop 92.

As shown, the preferred display circuit 62 could, for example, include aplurality of LEDs 114 corresponding to the number of steps in thedisplay scale 14 and at least one linear programmable chip 116, such asLM 3914, which has been appropriately programmed in a known manner.However, it will be understood that the display circuit 62 and thedisplay scale 14 could be alternatively constructed as any other desireddisplay means, such as an analog type dial meter. Also, the displayscale 14 could include multiple light bars corresponding to the numberof harmonics measured, and the main circuit 4 (including the displaycircuit 62) could be modified to provide a simultaneous display of eachof the measured harmonics as a percentage of the fundamental.

Referring to FIGS. 5a and 5b there is shown a power supply circuit 6 anda low battery indicator circuit 8 according to the present invention.The power supply circuit 6 preferably includes the pair of rechargeablebattery packs 28, 29, as discussed above, as well as a recharger circuit114 for the battery packs. The battery packs 28, 29 preferably havedifferent voltage outputs. For example, battery pack 28 could have a +12volt DC output, while battery pack 29 could have a +5 V and a -5 volt DCoutput. The battery recharger circuit 114 has input terminals 116, 118which would be connectable to an appropriate power source, such as 120volts AC, through the recharger cord 20 when necessary.

The low battery indicator circuit 8 includes an amplifier 120 and a pairof LEDs 122, 124. LED 122 functions as a voltage reference, while LED124 lights to indicate low battery at indicator 18. As indicated, thelow battery indicator circuit 8 is preferably connected to battery pack28 at terminal 126.

As discussed above, the main circuit 4 of the harmonic distortionmeasuring instrument 1 permits the instrument to be used formeasurements on systems having a wide range of voltages (for example, upto 600 volts), and will provide accurate harmonic readings over a verywide range of line currents (for example, over a range of 10-1000 amps).The depicted main circuit to 4 could also be used to measure linecurrents in the range of 0-10 amps, but such measurements would not beas accurate as measurements in the range of 10-1000 amps. It will beunderstood, however, that the depicted main circuit 4 is merely apreferred embodiment, and that the circuit 4 could be modified foraccurate use in any voltage range and any current range.

Operation of the instrument 1 is as follows.

Initially, an operator would connect the current transformer 8 to themain circuit 4 by plugging it in to the input jack 32, and would thenclamp the current transformer 8 around a desired conductor, such as apower line. The current transformer must be connected to the input jack32 before it is clamped around the conductor, or a high voltage will bedeveloped on the transformer leads and this can present a severe shockhazard.

For making the actual measurements, all that an operator is required todo is turn on the instrument 1 using the on/off switch 10, and then turnthe harmonic selector switch 16 to the desired harmonic. Normally, theselector switch will initially be turned to the fundamental harmonic,whereby the display scale 14 should read 100% if there is sufficientcurrent in the conductor being measured. The operator then would turnthe selector switch to each other desired harmonic and could record theharmonic amplitude measurements indicated by the display scale 14.Subsequently, the operator would interpret the recorded data todetermine the sources of problems that have been encountered withparticular conductors.

An exemplary data record sheet, including exemplary data, is shown inTable 1.

In reference to Table 1, the auto-gain feature of the instrument 1 wasused to determine/record the amplitude of harmonic distortion in theconductor while a suspect harmonic source, such as an SCR drive, wasturned on and off. In this manner, the auto-gain feature is particularlyuseful in locating equipment that is causing (is the source of) harmonicdistortion. From the data in Table 1, the operator would be able totrace motor overheating and excessive bearing wear to the fifth harmoniccomponent in the motor current when the SCR drive is operated at highpower. The harmonic current causes the motor rotor to "cog" and has theeffect of hammering on the bearings. The extra current also increasesthe operating temperature of the motor.

Also, the operator would note the concentration of harmonic current inthe capacitor. Since capacitor impedance decreases with frequency, thepresence of harmonics causes a large increase in capacitor current,resulting in heat dissipation. When the SCR drive is at idle, thedecrease is manageable, but with the drive operating a high power, thecapacitor current is near the limit for both the capacitor and the fuse.This could explain a capacitor fuse blowing which was observed. With theSCR drive at high power, enough harmonic distortion exists in the lineto affect computer circuits.

On the basis of the data shown in Table 1, the operator would be able torecommend a desired course of action. In this case, a course of actionwould be recommended which would improve the overall power factor andreduce the harmonic interference. For example, a tuned fifth harmonicfilter or trap, sized to improve the power factor to the desired value,could be connected to the line at the SCR drive. This should reduce theharmonic currents at the small capacitors. If the problems with highcapacitor currents continue, as determined by a recheck with theinstrument 1, replacing these capacitors with a trap is anotherpossibility.

Harmonic distortion measurements obtained using the instrument 1 couldobviously be used for many other purposes. For example, detection of thesecond harmonic of electrical energy in a conductor can be used toindicate DC current components. Also, the harmonic levels in each leg ofa three-phase system could be measured separately to determine if thephases are equally loaded.

Although there has been described what is at present considered to bethe preferred embodiment of the present invention, it will be understoodthat the invention can be embodied in other specific forms withoutdeparting from the spirit or essential characteristics thereof. Forexample, the modified signal which is being provided to the displaycircuit 62 could also (or alternatively) be provided to a computerizedenergy management system which would automatically control the power inthe conductor to be within a desired range. Similarly, the modifiedsignal could also be provided to an appropriate alarm means, such as anaudio alarm, a visual alarm, etc., which would be activated (forexample) whenever a measured harmonic exceeded a predetermined maximum,percentage of the fundamental.

The described embodiment is, therefore, to be considered in all aspectsas illustrative, and not restrictive. The scope of the invention isindicated by the appended claims rather than the foregoing description.

                                      TABLE 1                                     __________________________________________________________________________    PLANT                                                                              XYZ Inc.  DATE                                                                              today                                                                             PAGE    1 OF   1                                       LOCATION Big City, USA                                                        GENERAL DESCRIPTION of plant, problem, etc. Since installation of             SCR adjustable speed drives on blower motors, computers have been             malfunctioning. Capacitor fuses are blowing and other motors                  throughout the plant are overheating. Motor bearings are showing              abnormal wear.                                                                SYSTEM NAMEPLATE VOLTAGE volts                                                                      480                                                                              RATED LOAD kva                                                                            1500                                     POWER FACTOR from utility bill                                                                      75%                                                                              LOAD CURRENT amps                                                                         NA                                       POWER FACTOR measured                                                                            MEASURING INSTRUMENT                                       MEASURING POINT (diagram or description)                                                           30 hp pump motor with 7.5 KVAR                                                power factor correction capacitor.                                            A 100 KVA adjustable speed drive                                              has been added to the line and                                                its effect is to be measured.                                               OVER                                                       __________________________________________________________________________    TEST NUMBER→                                                                           1a  1b  1c  2a  2b  2c                                        MEASURING POINT→                                                                       A   B   C   A   B   C                                         EQUIPMENT                                                                     other equipment +   +   +   +   +   +                                         30 hp pump motor                                                                              +   +   +   +   +   +                                         variable drive  +   +   +   -   -   -                                         POWER LEVEL→                                                                           70% 70% 70% 0   0   0                                         CURRENT amps→                                                                          26  24  32  22  23  9                                         VOLTAGE volts→                                                                         465 465 465 470 470 470                                                HARMONIC                                                                       2→                                                                           0   0   0   0   0   0                                                   3→                                                                           5   0   10  0   0   0                                         HARMONIC %                                                                              5→                                                                           30  20  85  10  0   25                                                  7→                                                                           5   0   20  5   0   10                                                  9→                                                                           5   0   10  0   0   0                                                  11→                                                                           10  5   25  0   0   0                                                  13→                                                                           0   0   0   0   0   0                                                  15→                                                                           5   0   10  0   0   0                                                  17→                                                                           5   0   10  0   0   0                                                  19→                                                                           0   0   0   0   0   0                                         PHOTO CODE→      C-70%       C-0%                                      MEASUREMENTS TAKEN BY M. Z. L., R. J.                                                                COMPANY Myron Zucker, Inc.                             __________________________________________________________________________

We claim:
 1. Apparatus for locating and measuring harmonic distortion ina conductor, comprising: first means detachably connectable to aconductor for providing an input signal indicative of electrical energyin said conductor; second means connected to said first means andreceiving said input signal therefrom, said second means providing amodified signal indicative of the strength of a single, predeterminedharmonic of said input signal; andthird means connected to said secondmeans for selectively varying the single, predetermined harmonic whichis indicated by said modified signal.
 2. Apparatus according to claim 1,wherein: said apparatus further comprises a fourth means connected tosaid third means and receiving said modified signal therefrom, saidfourth means providing a display of said modified signal.
 3. Apparatusaccording to claim 2, wherein: said apparatus further comprises a fifthmeans interconnected between said first and second means forautomatically controlling a gain of said input signal so that the saidapparatus does not have to be recalibrated prior to each use, and sothat input signal remains essentially constant despite variations of theelectrical energy in said conductor.
 4. Apparatus according to claim 3,wherein: said apparatus further comprises a sixth means connected toeach of said first, second, third, fourth and fifth means for supplyingpower thereto.
 5. Apparatus according to claim 1, wherein: said firstmeans comprises a current transformer adapted to be selectively clampedaround said conductor.
 6. Apparatus according to claim 1, wherein: saidsecond means comprises a filter; and the modified signal indicates thestrength of the single, predetermined harmonic relative to the strengthof a fundamental harmonic of said input signal.
 7. Apparatus accordingto claim 1, wherein: said second means comprises a multiple-orderprogrammable, switch-capacitor, band pass filter; and the modifiedsignal indicates the strength of the single, predetermined harmonicrelative to the strength of a fundamental harmonic of said input signal.8. Apparatus according to claim 1, wherein: said third means comprises afrequency synthesizer.
 9. Apparatus according to claim 8, wherein: saidfrequency synthesizer includes a harmonic selector switch adapted to bemanually set by an operator to different harmonics, a source clock, anda phase lock loop connected between the source clock and the harmonicselector switch.
 10. Apparatus according to claim 9, wherein: saidfrequency synthesizer further comprises a programmable logic arrayconnected between said selector switch and said phase lock loop. 11.Apparatus according to claim 2, wherein: said fourth means comprises amulti-step light bar and a plurality of LEDs associated with the lightbar.
 12. Apparatus according to claim 4, wherein: said sixth meanscomprises a rechargeable battery, a recharger circuit connected to thebattery, said recharger circuit being selectively connectable to anexternal power source, and an indicator circuit connected to the batteryfor indicating when said battery is running low on power.
 13. Apparatusaccording to claim 12, wherein: said indicator circuit comprises a firstLED functioning as a voltage reference, and the second LED which lightsto indicate low battery power.